An alternator which charges the battery in a vehicle causes a ripple to be superimposed on the DC battery voltage, the frequency of which ripple is related to the engine speed. There are prior-art tachometers that calculate engine speed by measuring the frequency of the ripple. To convert ripple frequency to RPM requires knowledge of the relationship between such frequency and the rotational speed of mechanical parts of the engine. For example, it is known that the alternator speed is approximately one to three times the speed of the crank shaft. However, to obtain precise RPM readings the exact relationship in a particular vehicle must be determined. Prior-art approaches to calibration have not been sufficiently accurate.
Also known are tachometers that calculate RPM based on mechanical oscillations in the engine. For example, it is known to measure the frequency of the exhaust. As each cylinder fires, a pulse is produced in the exhaust. Measurement of the frequency of these pulses yields information on the engine speed. In such a tachometer, a microphone is placed near the exhaust pipe. The microphone picks up the exhaust sounds and the tachometer uses the frequency of those sounds to calculate RPM. A second type of mechanical oscillation is vibration of the engine. An accelerometer is placed near or on the engine to measure the frequency of such vibration, which frequency is related to engine speed.
At low engine speed, true RPM is very accurately related to the frequency of mechanical oscillations. But, the higher the engine speed, the lower the accuracy.
It has become important to provide virtually instantaneous measurement of the engine speed. This is because government specifications in various countries require measurement of certain parameters as a function of engine speed. For example, in Germany, there are specifications for the opacity of engine emissions in the range of 1,000 RPM to 10,000 RPM. Currently available tachometers do not have the capability of instantaneously keeping track of such extreme variations in engine speed.